Method and apparatus for reduction of voltage potential spike during dechucking

ABSTRACT

Provided is a substrate dechucking system of a plasma processing chamber adapted to remove a substrate from an ESC with reduction in voltage potential spike during dechucking of the substrate.

BACKGROUND

Electrostatic chucks are used to hold semiconductor wafers and othersubstrates during processing such as plasma etching. An electrostaticchuck (ESC) uses an electrostatic potential to hold (clamp) a substratein place during processing. By clamping the substrate to the chuck, ahigh thermal conductivity gas such as helium (He) may be disposedbetween the substrate and the chuck in order to improve heat transferbetween the substrate and the chuck. The substrate is removed from theESC by lift pins and a transfer arm removes the substrate from theprocessing chamber.

A difficulty that arises with the use of an ESC is the need for removalof the residual electrostatic force between the substrate and the chuckin order to remove the substrate from the chuck. This residual forceresults from the accumulation of electric charge at the interfacebetween the substrate and the ESC support surface. Several techniqueshave been developed for removing or de-chucking a substrate. Forexample, the electrode can be grounded or, alternatively, the polarityof the chucking voltage applied to the electrode can be reversed inorder to discharge the electrode. However, these techniques are notcompletely effective at removing all the charge on the electrode and thesubstrate. A mechanical force is often required to overcome the residualattractive electrostatic force, which can damage the substrate or createdifficulty in retrieving the substrate from an unintended position.Further, undesired particles can be generated during the substratedechucking and removal process which contaminate the processedsubstrate.

Despite the developments to date, there is an interest in apparatus andmethods which reduce any voltage potential spike during dechucking ofprocessed substrates.

SUMMARY

Provided is a substrate dechucking system of a plasma processing chamberadapted to remove a substrate from an ESC with reduction in voltagepotential spike during dechucking of the substrate.

In a method of dechucking a substrate from an electrostatic chuck in aplasma processing chamber, a process gas is supplied into the chamberand energized into a plasma state, the plasma chamber is maintained at avacuum pressure and low RF power to produce a plasma sheath above thesubstrate of 2 mm or greater thickness, the substrate is lifted above asupport surface to a mid-lift position within the plasma sheath whichdoes not induce plasma instability and maintained there, the plasma isextinguished, and the substrate is lifted above the mid-lift position toan upper position at which the substrate can be removed from the plasmachamber.

In another embodiment, a pneumatic lift mechanism for a plasma reactoris provided wherein lift pins raise and lower a substrate in at leastthree positions with respect to an electrostatic chuck. The liftmechanism preferably comprises vertically aligned pneumatically operatedupper and lower pistons, wherein the upper piston is slidably mounted tomove up and down in an upper chamber and the lower piston is slidablymounted to move up and down in a lower chamber, and the lower chambercomprises a hard stop defining an upper limit of travel of the lowerpiston. The at least three positions preferably comprise (1) a lowerposition at which the upper and lower pistons are in down positions; (2)a mid-lift position at which the lower piston is at an upper position incontact with the hard stop and a shaft extending upward from the lowerpiston contacts the upper piston to partially raise the upper piston;and (3) an up position at which the upper piston is an upper positionand a substrate supported on lift pins driven by the upper piston can beremoved from the plasma chamber. The upper piston includes an uppershaft which cooperates with a yoke driving lift pins to (1) lower asubstrate onto a substrate support when the upper and lower pistons arein the lower position, (2) raise the substrate to a mid-lift positionwhen the pistons are in the mid-lift position, and (3) raise thesubstrate to an upper position at which it can be removed by a transferarm when the upper piston is in the upper position.

In a preferred method, the process chamber is a plasma etch chamber andthe processing comprises generating a plasma adjacent the upper surfaceof the substrate and etching an exposed layer on the upper surface ofthe substrate with the plasma. Alternatively, the processing cancomprise forming a layer on the upper surface of the substrate (e.g., bychemical vapor deposition, thermal oxidation, sputtering or otherdeposition processes). Still yet, the processing can comprise strippingphotoresist or other material from the substrate.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a side elevation view, in cross section, of a plasma reactorin accordance with one embodiment.

FIG. 2A is a depiction of a substrate pin-lift system wherein the systemis supported on an ESC.

FIG. 2B is a depiction of the pin-lift system of FIG. 2A wherein thesubstrate has been lifted above the ESC.

FIG. 3A is a depiction of exemplary pneumatic lift hardware in a downposition. FIG. 3B is a depiction of exemplary lift hardware in amid-lift position. FIG. 3C is a depiction of exemplary lift hardware inan up position.

FIG. 4 is a schematic of an exemplary control system for pneumatic lifthardware.

FIG. 5 shows substrate voltage when dechucked using various methods.

FIGS. 6A and 6B show substrate voltage when dechucking a substrate usingvarious dechucking voltages, without and with a plasma-on dechuckingstep, respectively.

FIG. 7 shows particle contamination levels of substrates dechucked usingvarious methods including or excluding helium backpressure and a midliftplasma-on step.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As used herein, the term “about” when used in conjunction with a statednumerical value or range denotes somewhat more or somewhat less than thestated value or range, to within a range of ±10% of that stated.

A method of dechucking that is capable of reducing voltage potentialspikes during dechucking from an ESC is described herein.

The substrate can comprise a semiconductor wafer used to manufactureintegrated circuits, a substrate for 3-D chip integration, a glasssubstrate used to manufacture a flat panel display, or a silicon waferbonded to a glass carrier.

Preferred embodiments are practiced in conjunction with a plasmareactor, such as a capacitively coupled plasma reactor, e.g., an Exelan™plasma etcher, which is available from Lam Research Corporation ofFremont, Calif.

A preferred plasma reactor comprises a dual frequency capacitivelycoupled plasma reactor including an upper showerhead electrode and abottom electrode, RF energy being supplied at two different frequencies(e.g., 27 MHz and 2 MHz) to the bottom electrode. See, for example,commonly-owned U.S. Pat. No. 6,391,787, the disclosure of which ishereby incorporated by reference in its entirety. In the case where theplasma reactor is a capacitively coupled plasma etch reactor, the bottomelectrode is preferably supplied about 500 to 3000 Watts of RF energy,with optionally a lower wattage of 50 Watts during a dechuckingoperation.

Other preferred embodiments are practiced in conjunction with aninductively coupled plasma reactor. See, for example, commonly-ownedU.S. Pat. No. 7,223,321, the disclosure of which is hereby incorporatedby reference in its entirety. In operation of such a reactor, a reactantgas flows into a chamber and high frequency power is applied by an RFpower supply to a coil, generating an electromagnetic field about thecoil. The electromagnetic field is inductively coupled into the chamberand energizes reactant gas into a plasma.

FIG. 1 illustrates a plasma reactor in accordance with one exemplaryembodiment. Additional details regarding such a plasma reactor may befound in commonly-assigned U.S. Patent Publication No. 2008/0318433,which is incorporated herein by reference. It should be understood,however, that various configurations of the process chamber and internalcomponents, including the lower and upper electrodes, chamber walls andprocess gas distribution system, may be used. See, for example,commonly-owned U.S. Pat. Nos. 6,824,627 and 7,428,550, each of which isincorporated herein by reference.

In FIG. 1, a capacitively-coupled plasma processing chamber 100 has aplasma confinement ring assembly 10 mounted therein. The plasmaprocessing chamber 100 includes an upper electrode 102 having a bottomsurface 104. In the embodiment, the bottom surface 104 includes a step106 adapted to control a localized density of the plasma formed adjacentthe exposed surface of the upper electrode 102, as described in U.S.Pat. No. 6,391,787, which is incorporated herein by reference in itsentirety. In the embodiment, the upper electrode 102 is a showerheadelectrode including gas passages 108 arranged for distributing processgas into the plasma processing chamber 100. The upper electrode 102 canbe comprised of silicon (e.g., single crystal silicon or polycrystallinesilicon) or silicon carbide.

In the embodiment, the upper electrode 102 is a single-piece electrode(e.g., for 200 mm wafer processing). The upper electrode 102 ispreferably mounted (e.g., elastomer bonded) to a backing member 110 of asuitable material, such as graphite or silicon carbide. The backingmember includes gas passages 112 in fluid communication withcorresponding gas passages 108 in the upper electrode 102.

The upper electrode can be a planar electrode or a non-planar, steppedupper electrode such as a showerhead electrode as disclosed incommonly-owned U.S. Pat. No. 6,391,787. The RF electrodes can be madefrom any suitable electrically conductive material. For example, theupper electrode can comprise high purity, low resistivity single crystaland the lower electrode can comprise a metal such as silicon (e.g.,doped silicon), aluminum and the like.

In another embodiment, the upper electrode can have a two-piece orsegmented construction (e.g., for larger wafer processing, such as 300mm wafers) and include a single-piece inner electrode member and anouter electrode member surrounding the inner electrode member, such asdescribed in commonly-owned U.S. Patent Application Publication No.2005/0133160, which is incorporated herein by reference in its entirety.In the embodiment, the backing member preferably includes a backingplate co-extensive with the inner electrode member and a backing ringco-extensive with the outer electrode member, as described in U.S.Patent Application Publication No. 2005/0133160.

In the embodiment of the plasma processing chamber 300 shown in FIG. 1,a thermal control plate 114 is preferably provided on the backing member110. The thermal control plate 114 preferably includes one or moreheaters adapted to control the temperature of the upper electrode 102,as described in commonly-owned U.S. Patent Application Publication No.2005/0133160, incorporated herein by reference.

The plasma processing chamber 100 includes a gas source (not shown) forsupplying process gas to the upper electrode 102. The process gas isdistributed in the chamber by the gas passages 108 in the upperelectrode 102. The upper electrode 102 can be powered by an RF powersource 116 via a matching network. In another embodiment, the upperelectrode 102 can be electrically grounded to provide a return path forpower supplied by a bottom electrode of the substrate support 120 of theplasma processing chamber 100.

In the embodiment, process gas is supplied into the plasma processingchamber 100 at the plasma generation region in the space between theupper electrode 102 and a semiconductor substrate 122, e.g., asemiconductor wafer, supported on a substrate support 120.

In addition to a semiconductor wafer, the substrate 122 can comprise aglass panel to be processed into a flat panel display, or a siliconwafer carried by a glass substrate to be processed (e.g., for 3-D chipintegration). The substrate 122 can comprise one or more layers to beselectively removed (etched) during processing or, alternatively, theprocessing can comprise forming one or more layers on the substrate orother process such as photoresist stripping.

The substrate support 120 preferably includes an electrostatic chuck 124that secures the semiconductor substrate 122 on the substrate support byan electrostatic clamping force. The electrostatic chuck 124 can beincorporated in or mounted on a bottom electrode (also called a lowerelectrode) and can be powered by at least one of the RF power sources126, 127 (typically via a matching network).

The lower electrode can be used to supply RF power to produce a plasmafrom process gas in the gap above the substrate and optionally apply anRF bias to the substrate. The amount of energy that is coupled betweenthe upper and/or lower electrode and the plasma generally affects thedensity and energy of the plasma used to process the substrate. Forexample, if the coupled energy is large, the ion energy tends to behigh. If the coupled energy is small, the ion energy tends to be low.Correspondingly, high ion energy tends to be more aggressive duringsubstrate processing and a low ion energy tends to be less aggressiveduring substrate processing. The energy generated by the bottomelectrode may also be arranged to form a sheath voltage proximate to thesubstrate surface, which is used to accelerate the ions in the plasmatowards the substrate where they can activate the processing reaction.

Preferably, the outer periphery of the lower electrode is configured toextend beyond at least the outer edge of the wafer with an edge ringarranged above the lower electrode and surrounding the wafer.

During plasma processing of the semiconductor substrate 122, the plasmaconfinement ring assembly 10 confines plasma in a plasma confinementzone between the upper electrode 102 and the semiconductor substrate122. Edge rings 126, 128 are preferably arranged in surroundingrelationship to the semiconductor substrate 122 to focus the plasma soas to improve etch uniformity.

A vacuum pump (not shown), preferably a turbomolecular pump, is adaptedto maintain a desired vacuum pressure inside the plasma processingchamber 300.

An exemplary parallel-plate plasma reactor that can be used is adual-frequency plasma etch reactor (see, e.g., commonly-owned U.S. Pat.No. 6,090,304, which is hereby incorporated by reference in itsentirety). In such reactors, etching gas can be supplied to a showerheadelectrode from a gas supply and plasma can be generated in the reactorby supplying RF energy at different frequencies from two RF sources tothe showerhead electrode and/or a bottom electrode. Alternatively, theshowerhead electrode can be electrically grounded and RF energy at twodifferent frequencies can be supplied to the bottom electrode.

To process a substrate, the substrate is loaded into the chamber, andplaced on the support surface of the lower electrode. For example, arobot arm (not shown) can transport a substrate from a load-locktransfer chamber into the process chamber. A lift pin assembly (notshown) has lift pins that can be raised and lowered by a lift mechanism.Preferably the lift pins are electrically and thermally insulated fromthe lower electrode. Preferably the lift pins are made of sapphire, butcan be metallic or dielectric. The robot arm can place the substrate onthe tips of the lift pins and the lift mechanism can lower the substrateonto the support surface. After processing the substrate, the liftmechanism can raise the lift pins to lift the substrate off the lowerelectrode, allowing the substrate to be removed from the processingchamber via the robotic arm.

The lift pins can be raised and lowered by a pin lifter yoke such asthat described in commonly-owned U.S. Pat. No. 6,646,857, the disclosureof which is hereby incorporated by reference. Alternatively, the lowerelectrode can also include lift-pins such as cable actuated lift pinsmovable towards and away from the support surface such that thelift-pins travel through holes in the lower electrode to raise and lowera substrate. A cable-actuated drive assembly for moving a substrate in avacuum chamber is disclosed in commonly-owned U.S. Pat. No. 5,796,066,the disclosure of which is hereby incorporated by reference in itsentirety. The number of lift pin holes generally depends on the size ofthe substrate. In a preferred embodiment, the lift pins are actuated bya gas, for example by using pneumatic lift hardware.

According to a preferred embodiment, a method of etching a substrateincludes supporting the substrate on the lower electrode, supplyingprocess gas to the chamber and energizing the process gas into a plasmaand etching an exposed surface of the substrate with the plasma. Theprocess is applicable to etching of various silicon and/or dielectriclayers including low-k dielectric material such as doped silicon oxidesuch as fluorinated silicon oxide (FSG), undoped silicon oxide such assilicon dioxide, spin-on-glass (SOG), silicate glasses such as boronphosphate silicate glass (BPSG) and phosphate silicate glass (PSG),doped or undoped thermally grown silicon oxide, doped or undoped TEOSdeposited silicon oxide, etc. Other materials that may be etched includeorganic low-k material such as BLACK DIAMOND available from AppliedMaterials, Inc. and the CORAL family of low-k films available fromNovellus. The dielectric dopants include boron, phosphorus, and/orarsenic. The dielectric can overlie a conductive or semiconductive layersuch as polycrystalline silicon, metals such as aluminum, copper,titanium, tungsten, molybdenum or alloys thereof, nitrides such astitanium nitride, metal silicides such as titanium silicide, cobaltsilicide, tungsten silicide, molybdenum silicide, etc.

On 2300 Exelan™ systems, an ESC is used to hold silicon wafers on atemperature controlled bottom electrode during an oxide etch process.Wafer temperature control is effected by pressurized helium (He)supplied between the ESC upper surface and lower surface (backside) ofthe wafer. Details of a system for controlling helium backside coolingof wafers can be found in commonly-owned U.S. Pat. No. 6,140,612, thedisclosure of which is hereby incorporated by reference. While He couldbe used to dechuck the wafer after the clamping voltage is no longerapplied, He flowed during and immediately after dechucking can causeparticles to migrate from the He interface between the ESC and backsideof the wafer to the plasma etched surface and cause device damage tointegrated circuitry fabricated on the frontside of the wafer.

To mitigate particle contamination of plasma processed substrates, thewafer can be exposed to plasma gas, which is believed to help minimize achange in substrate voltage potential during subsequent lifting of thesubstrate for removal from the plasma processing chamber. One method toaccomplish this uses mechanical lift pins to partially raise the waferto a mid-lift position, thus positioning the wafer at least partiallywithin a plasma sheath.

Optionally, low helium backpressure (e.g., 5 Torr or less) can beapplied to slightly separate the wafer while the dechucking plasma ison, but turned off before the plasma is extinguished. If the He remainsapplied while the plasma is extinguished, backside particles can betransported to the front side of the wafer by helium flow.

In an embodiment, after voltage to the ESC is turned off and the supplyof He to the He interface is terminated, lift pins are raised to liftthe substrate off of the ESC while plasma is maintained in the chamber.The substrate is maintained in the plasma sheath while preventingpenetration of the plasma beneath the wafer and preventing plasmainstability.

Subsequently, the plasma is extinguished and the substrate is raised toan upper position at which the substrate can be transferred out of thechamber.

Although described in embodiments, no force sensor or application ofdechucking voltage is required for dechucking. However, if desired,either or both may be employed.

In accordance with a preferred embodiment, a method of dechucking asubstrate from an electrostatic chuck in a plasma processing chambercomprises: supplying process gas into a gap between an upper electrodeand a bottom electrode on which the substrate is held against a supportsurface by the electrostatic chuck, energizing the process gas in thegap into a plasma state by supplying radio frequency power (for example,to a lower electrode or a coil); maintaining the plasma chamber at avacuum pressure which provides a plasma sheath above the lowerelectrode; lifting the substrate above the support surface to a mid-liftposition within the plasma sheath which does not induce plasmainstability or allow plasma penetration beneath the wafer; maintainingthe substrate at the mid-lift position for a suitable time such as 2 to30 seconds; and lifting the substrate above the support surface to anupper position at which the substrate can be removed from the plasmachamber.

In accordance with various optional features of the method, the processgas can be argon, nitrogen or a mixture thereof. The substrate ispreferably lifted by lift pins made of electrically insulating material(for example, sapphire) which lift the substrate 0.5 to 3 mm (e.g., 1.5to 2 mm) above the support surface of the electrostatic chuck in themid-lift position. The substrate can be a wafer of silicon or othermaterial and may have at least one layer of dielectric material.

Prior to lifting the substrate, voltage to the electrostatic chuck ispreferably set to a predetermined value relative to the bias expected tohave been induced on the wafer by the plasma, and a supply of He gas tothe underside of the substrate is terminated. The gap between the upperand lower electrodes can be any suitable distance such as at least 20 mmand vacuum pressure in the chamber can be any suitable value such as avalue in the range of 15 to 500 mTorr, for example up to 25, 50, 100,150, 200, 250, or 300 mTorr. When the substrate is lifted to the upperposition, the substrate is preferably lifted to at least 9 to 15 mm(e.g., 9.5 to 12.5 mm) above the support surface.

FIG. 2A shows a cross sectional view of a semiconductor wafer liftingdevice which can be used to carry out the wafer dechucking methoddescribed above. Additional details regarding such a device may be foundin commonly-assigned U.S. Pat. No. 6,646,857, the entirety of which isincorporated herein by reference. The semiconductor wafer lifting deviceincludes a number of lifting pins 228 which traverse electrostatic chuck204 through a suitable number of penetrations 226 depending on the sizeof the substrate. Typically, there are either 3 or 4 lifting pins 228which are equidistant apart and connected to lifting yoke 230. The spacebetween the lifting pins 228 and the electrostatic chuck 204 is isolatedfrom the space below the chuck by bellows 232 and sealant rings 234. Useof bellows 232 allows the yoke 230 to move with respect to the chuck 204without compromising the atmospheric isolation within the processingchamber. A strain gauge 242 is positioned between insulating material240 and a lead screw 244 which is driven by a motor 246 to move the yoke230. It should be noted that the lead screw 244 can be replaced with anytype of link, such as a pneumatic lift mechanism, so long as it is ableto raise and lower the pin lifter yoke 230.

The strain gauge 242 sends information signals to a controller such as adigital signal processor (DSP) 250, which in turn sends signals to amotor controller 252, which in turn sends signals to the motor 246 forcontrolling the positioning of the lead screw 244. An encoder 248 isinterfaced with the motor 246 and is configured to send signals to themotor controller 252. The information provided by the encoder 248 willcontain current position data for the lead screw 244.

The lifting pins 228 are configured to contact the underside of thewafer 206 and apply a low force, especially during the mid-lift process,in order to prevent the application of high stresses that can have thepotential of damaging the wafer while it is electrostatically clamped tothe chuck 204.

The lifting yoke 230 can be moved incrementally upward to lift the wafer206 from the chuck 204. As the lifting pins 228 experience forcesresistant to lifting of the wafer 206, these forces will be transferredthrough the lifting yoke 230 to the optional strain gauge 242. Thestrain measurement from the strain gauge 242 will in turn be monitoredby the DSP 250. If the monitored strain force is not within anacceptable range, the DSP 250 (and associated software, if necessary)will instruct the motor controller 252 to stop incrementing the motorand timeout to allow electrostatic discharge of the wafer 206, such thatelectrostatic attraction between the wafer 206 and chuck 204 may befurther reduced. The stopping of the incrementing occurs when athreshold value is reached. The threshold value is preferably a setvalue that identifies when the force applied by the lift pins hasreached a level that might cause damage to the wafer. Thus, thethreshold value is just below that level that might cause wafer damage,and in this manner, the wafer is protected from excessive forces whensufficient discharge has not taken its course.

The acceptable force limit under gravity can range between about 1 ounceand about 5 pounds, depending on the size of the wafer. If the monitoredforce is within an acceptable range, the DSP 250 and associated softwarewill instruct the motor controller 252 to continue incrementing themotor. During the incrementing, the encoder 248 attached to the motor246 will monitor the position of the lead screw 244 and signal the leadscrew position to the motor controller 252. The motor controller 252will then instruct the motor 246 to stop lifting the yoke 230 when thedesired lifting height is attained.

FIG. 2B shows a cross sectional view of a semiconductor wafer liftingdevice as the lifting pins 228 lift the wafer 206. The components shownin FIG. 2B are identical to those shown and described in FIG. 2A. FIG.2B illustrates the movement of the lifting yoke 230 as the wafer 206 islifted off of the chuck 204 to the mid-lift position and then to theupper position as discussed above. At this point, the wafer 206 will beready to be picked off of the lifting pins 228 by the blade of a robot.Now, another wafer 206 can be placed on the lifting pins 228 so that itcan be lowered onto the chuck 204 for processing.

In an alternative embodiment, a pneumatic pin lifter can be used inplace of the motor 246 and lead screw 244 to control the stroke of thepin lifting mechanism. For example, a drive system operated by aseparate pressurized gas source (exemplary gases including air, argon,helium, and nitrogen) could be used to move the pin lifter yoke and liftpins to an upper position at which the substrate can be transferred ontoand removed from the lift pins by a transport arm that transfersubstrates into and out of the plasma chamber, a lower position at whichthe substrate can be clamped by the ESC, and a mid-lift position atwhich the substrate is held within the plasma sheath. Lifting can beaccomplished by the use of lift hardware as described below withreference to FIGS. 3A-C.

A pin-lift mechanism can be operated under the control of a controllerwhich monitors the position of the lift pins and directs the pin liftmechanism to raise or lower the lift pins to at least upper (alsoreferred to as “full up” or simply “up”), mid-lift, and lower positions.Preferably, a lift mechanism includes one or more position sensors, morepreferably optical sensors to detect when the mechanism is in mid-liftand up positions. Such optical sensors may detect movement of the yokeor of one or more flags affixed to the yoke.

An exemplary lift mechanism for use in a plasma reactor is depicted inFIGS. 3A, 3B, and 3C. Such a mechanism may replace the motor andassociated components in FIGS. 2A and 2B. An upper piston 301 and alower piston 302 are slidably mounted in an upper chamber 304 and alower chamber 305, respectively, all in a housing 303. The upper piston301 includes a vertically extending upper shaft 311 which is preferablyoperably connected to a pin lifter yoke (not shown). The lower piston302 includes a vertically extending lower shaft 312 that may act topartially raise the upper piston. The chambers 304 and 305 arepreferably stationary with respect to the ESC due to fixedly mountinghousing 303. Such a lift mechanism does not require the use of a straingauge or the like (such as a force sensor), however one may be used, forexample between the upper shaft 311 and a pin lifter yoke.

Actuation of the lift mechanism to a down position, as shown inschematic form in FIG. 3A, is accomplished by applying gas pressure(e.g., 70 to 120 psig, preferably 90 psig) through a first inlet 306 topressurize an upper portion of the upper chamber 304. This pressureexerts a downward force against the upper piston 301, and forces theyoke and its lift pins to a down position with the upper ends of thelift pins below the ESC upper surface. Preferably, 90 psig or a lowerpressure such as 50 psig is also applied through inlet 308 to the top oflower chamber 305, to ensure that the lower piston 302 does not lift theupper piston 301.

Actuation to a mid-lift position, as shown in schematic form in FIG. 3B,is accomplished by applying gas pressure (e.g., 25-65 psig, preferably50 or 60 psig) through first inlet 306 to the top of upper chamber 304while applying higher gas pressure (e.g., 70 to 120 psig, preferably 90psig) to fourth inlet 309 to the bottom of lower chamber 305. The lowerpiston 302 is thus raised a predetermined distance based upon locationof a hard stop 310 at the top of lower chamber 305, where preferably thehard stop 310 has been pre-adjusted in order desirably determine themid-lift position. The lower shaft 312 of lower piston 302 in turnpushes upper piston 301 to a corresponding partially raised position sothat a wafer is raised to a mid-lift position above the ESC so as to bewithin a plasma sheath without inducing plasma instability. A thirdinlet 308 may vent the top of lower chamber 305 and, optionally in alowering step, be pressurized to lower the lower piston 302 prior tolowering the upper piston 301.

Actuation to a full up position (to facilitate removal of a wafer), asshown in schematic form in FIG. 3C, is accomplished by applying gaspressure (e.g., 60 to 120 psig, preferably 90 psig) through second inlet307 to a lower portion of top chamber 304 while applying 50 psig (oroptionally 65, 70, or 75 psig) through a first opening 306 to pressurizean upper portion of a top chamber 304, thereby raising upper shaft 301.

The opposing pressure applied to the top of the upper chamber duringmid-lift and full-up actuation is used to limit max up force in theevent that the wafer is not well dechucked (for example, if the waferexhibits an unexpectedly high attraction to the chuck) and thus minimizerisk of breaking a wafer. The opposing pressure also reduces suddenforces on the wafer which could dislodge the wafer from a desiredpositional relationship with the lift pins. Optionally, the forceapplied to the lift pins may be used as an indicator that a dechuck hasbeen performed.

Preferably, the mid-lift position raises the wafer about 0.5 to 3 mm(e.g., 2 mm to 3 mm) above the ESC and the up position raises the waferabout 9.5 mm (0.375 inches) to 12.5 mm (0.433 inches) above the ESC.

In a preferred embodiment, the pressures used to supply the liftmechanism come from supply lines, for example separate 50 psig (oroptionally 65, 70, or 75 psig) and 90 psig gas supply lines. Supply ofthe gas to inlets 306-309 may be controlled by electronically-controlledon-off valves. Preferably, such valves are activated to open or closedpositions by a controller, for example a controller that also controlsother operational aspects of the plasma reactor.

FIG. 4 shows a schematic of an exemplary control system for pneumaticactuator hardware. A pneumatic actuator 400 has its second throughfourth inlets 307, 308, and 309 each connected to a pneumatic bank 410and its first inlet 306 connected to shuttle valve 401. The pneumaticbank comprises controllable valves. It receives higher pressure gas(e.g., 60 to 120 psig, preferably 90 psig) from a higher pressure gassupply 420 and is controlled by sense and control logic controller 440to supply gas as desired to the inlet ports 307, 308, and 309 and toshuttle valve 401. The shuttle valve 401, in addition to receiving gaswhen sent from the pneumatic bank, is also connected to receive gas froma lower pressure gas supply 430 (which provides 50 psig, or optionally65, 70, or 75 psig). The shuttle valve 401 preferably operates toautomatically supply the higher of the received pressures to first inlet306 (i.e., it supplies the lower pressure gas by default, or the higherpressure gas when the pneumatic bank 410 supplies the higher pressuregas to the valve 401), or less preferably the valve 401 can becontrolled independently from bank 410. The sense and control logiccontroller 440 may receive inputs from optional sensors (not shown), forexample to detect the position of a yoke.

A lift mechanism for implementing a mid-lift step may be used with orwithout a dechucking voltage to the ESC and wafer and with or withoutsupplying He gas to the underside of the wafer. If a dechucking voltageis used, the voltage is preferably set to a value within 200V of thebias voltage induced on the wafer by the plasma prior to lifting thewafer, e.g., −200 V to +200 V, including zero. For a 300 mm EXELAN, thepreferred dechucking voltage is −50V for a 50 W 27 MHz plasma at 50mTorr, as is within about 20V of the measured bias voltage on the wafer.The use of a dechucking voltage alone (without a plasma-on substratedischarging step, for example as part of a mid-lift step) may presentdifficulties, in that polarity and magnitude of a dechucking voltagesufficient to eliminate residual charge on the substrate varies with thetype of substrate, substrate temperature, dechuck recipe, etc., so thatdetermination of a dechucking voltage optimal to eliminate substratecharge must be repeated each time reactor process conditions change,which is time-consuming. Unless the substrate charge is neutralized, thedechucking process may cause the voltage on the substrate to spikeduring wafer lift, which could electrostatically attract contaminants toadhere to the substrate.

A helium backpressure of 1 to 10 Torr on the underside of the wafer canbe used to slightly lift the wafer to a mid-lift position wherein thewafer is at least partially within the plasma sheath. If heliumbackpressure is used, a preferred pressure is 2-5 Torr, preferably 3Torr. However, in a preferred embodiment, 0 Torr helium backpressure isused in the dechuck step prior to a plasma-on midlift of the wafer.

FIG. 5 shows test results of wafer voltage during dechucking from anESC. The line of alternating dots and dashes shows wafer voltage for aplasma dechuck at 0 Torr helium backside pressure followed by aplasma-off lift (labeled “0 T He dechuck”); the dotted line shows wafervoltage for a plasma dechuck with 3 Torr helium backside pressurefollowed by a plasma-off lift (labeled “3 T He dechuck”), and the solidline shows wafer voltage for a plasma dechuck at 0 Torr helium backsidepressure followed by a plasma-on mid-lift dechucking step wherein thewafer is lifted above a support surface of the ESC to a mid-liftposition within a plasma sheath (labeled “Middle Lift dechuck”). Notethat “Pins Up” times are staggered in the chart for readability. TheTables below provide additional details regarding the parameters underwhich these tests were performed.

It can be seen that the use of a plasma-on mid-lift to a wafer positionwithin the plasma sheath minimized the voltage spike seen by the waferduring the lifting of the wafer. In contrast, if the RF power is turnedoff before the lifting of the wafer, a large voltage spike occurs. Usinga helium 3 T backpressure during the dechuck plasma reduces the size ofthe voltage spike compared to no backpressure, when the wafer liftoccurs with no plasma.

Table 1 below shows plasma chamber operational parameters used in thetest of the plasma-on mid-lift step during dechucking. In this Table andthe other tables, “ME” represents “main etch,” “DCProbe” refers tomeasured wafer bias voltage (which can be used to set the voltageapplied to the ESC), “DC1” represents “Dechuck 1” (an initial reducedpower setting at the start of the dechuck process, allowing the plasmareactor to stabilize), and “DC2” represents “Dechuck 2” (showing powersettings during dechucking).

TABLE 1 Mid-lift dechuck. Step Description Stability Strike ME DC1 DC2MidLift RF Off Pressure (mtorr) 120 120 100 0 50 50 0 RF 2 MHz Power (w)0 50 100 1 0 0 0 RF 27 MHz Power (w) 0 50 100 50 50 50 0 Step Type StabTime Time Time Time Time Time Process Time (sec) 20 5 25 4 16 10 2 C₄F₈(50.0 sccm) 13 13 13 0 0 0 0 CO (510.0 sccm) 65 65 65 0 0 0 0 O₂ (52.0sccm) 8 8 8 0 0 0 0 Ar (1010.0 sccm) 800 800 800 400 400 400 400 HeliumInner Zone (torr) 30 30 30 30 0 0 0 Helium Outer Zone (torr) 30 30 30 300 0 0 ESC Bias Mode Fixed Fixed DCProbe DCProbe Fixed Fixed DCProbe ESCBias Voltage (±Vdc) 200 200 0 0 −50 −50 0 ESC Full Time Bias Comp EnableEnable Disable Enable Enable Enable Enable Lifter Pin Position down downdown down down middle middle

Table 2 below shows plasma chamber operational parameters used in the 0T He dechuck test, without a plasma-on mid-lift step.

TABLE 2 0T He dechuck. Step Description Stability Strike ME DC1 DC2Delay MiddleLift Pressure (mtorr) 120 120 100 0 50 0 0 RF 2 MHz Power(w) 0 50 100 1 0 0 0 RF 27 MHz Power (w) 0 50 100 50 50 0 0 Step TypeStab Time Time Time Time Time Time Process Time (sec) 20 5 25 4 16 2 2C₄F₈ (50.0 sccm) 13 13 13 0 0 0 0 CO (510.0 sccm) 65 65 65 0 0 0 0 O₂(52.0 sccm) 8 8 8 0 0 0 0 Ar (1010.0 sccm) 800 800 800 400 400 0 400Helium Inner Zone (torr) 30 30 30 30 0 0 0 Helium Outer Zone (torr) 3030 30 30 0 0 0 ESC Bias Mode Fixed Fixed DCProbe DCProbe DCProbe DCProbeDCProbe ESC Bias Voltage (±Vdc) 200 200 0 0 0 0 0 ESC Full Time BiasComp Enable Enable Disable Enable Enable Disable Enable Lifter PinPosition down down down down down down middle

Table 3 below shows plasma chamber operational parameters used in the 3T He dechuck test, without a plasma-on mid-lift step.

TABLE 3 3T He dechuck. Step Description Stability Strike ME DC1 DC2Delay MiddleLift Pressure (mtorr) 120 120 100 0 50 0 0 RF 2 MHz Power(w) 0 50 100 1 0 0 0 RF 27 MHz Power (w) 0 50 100 50 50 0 0 Step TypeStab Time Time Time Time Time Time Process Time (sec) 20 5 25 4 15.5 2 2C₄F₈ (50.0 sccm) 13 13 13 0 0 0 0 CO (510.0 sccm) 65 65 65 0 0 0 0 O₂(52.0 sccm) 8 8 8 0 0 0 0 Ar (1010.0 sccm) 800 800 800 400 400 0 400Helium Inner Zone (torr) 30 30 30 30 3 3 0 Helium Outer Zone (torr) 3030 30 30 3 0 0 ESC Bias Mode Fixed Fixed DCProbe DCProbe DCProbe DCProbeDCProbe ESC Bias Voltage (±Vdc) 200 200 0 0 0 0 0 ESC Full Time BiasComp Enable Enable Disable Enable Enable Disable Enable Lifter PinPosition down down down down down down middle

FIG. 6A shows that the wafer voltage potential spike at pins up can becontrolled for a plasma-off lift by adjusting the dechuck voltage. Thetwo dark solid lines (a thick line and a thin line) show the results oftwo runs where the dechuck voltage was −50V, which resulted in largepositive voltage spikes at dechuck. The lighter-color solid line and theline composed of long dashes show the results of two runs where thedechuck voltage was 0V, which resulted in large negative voltage spikesat dechuck. The three other lines (composed of short dashes, dots, andstars) show the results of three runs where the dechuck voltage was−25V, which resulted in smaller voltage spikes at dechuck as compared tothe other examples. By proper choice of the dechuck voltage applied tothe ESC, the wafer voltage spike at pins up can be minimized. Thesetests were performed with a plasma-on dechucking step followed by aplasma-off lift.

In contrast, FIG. 6B shows wafer voltage potential at various dechuckvoltages during dechuck with the plasma on during a mid-lift step. Inthis case, with plasma on during pins up, the wafer potential spike atpins up is more consistently low (<25V in magnitude) and relativelyinsensitive to the dechuck voltage value when compared to FIG. 6A. Thus,it is expected that a mid-lift step should be useful in reducing voltagespikes in a variety of situations which might require differing dechuckvoltages.

FIG. 7 shows particle contaminants measured on wafers dechucked with andwithout 3 T helium backpressure and a dechucking plasma on, but withouta plasma-on midlift step (columns A and B, respectively), or when usinga plasma-on mid-lift step in the dechuck process (column C). Using themidlift plasma dechuck (with 0 Torr He during the plasma dechuck andplasma-on midlift) recipe results in lower particle counts compared to 3Torr He plasma-off dechuck.

Lower particle counts with midlift plasma dechuck are partly explaineddue to reduced particle transport by electrostatic attraction to thewafer as a result of a lower voltage spike, and, in part, by thereduction of He flow transport of backside particles achieved by using 0Torr He backside pressure during the dechuck and lift. For example,negatively charged wafer backside particles and particles on theadjacent edge ring are less likely to be attracted to the wafer duringthe lift if positive wafer potential is minimized.

Although the invention has been described with reference to particularembodiments and examples, it should be understood that variousmodifications can be made without departing from the spirit of theinvention. The various parts of the disclosure including the abstract,summary, and the title are not to be construed as limiting the scope ofthe present invention, as their purpose is to enable the appropriateauthorities, as well as the general public, to quickly determine thegeneral nature of the invention. Unless the term “means” is expresslyused, none of the features or elements recited herein should beconstrued as means-plus-function limitations. Accordingly, the inventionis limited only by the claims.

1. A method of dechucking a substrate from an electrostatic chuck in aplasma processing chamber, the method comprising: supplying process gasinto the chamber; energizing the process gas into a plasma state;maintaining the plasma chamber at a vacuum pressure and low RF power toproduce a plasma sheath above the substrate of 2 mm or greaterthickness; lifting the substrate above a support surface of theelectrostatic chuck to a mid-lift position within the plasma sheathwhich does not induce plasma instability and maintaining the substrateat the mid-lift position; extinguishing the plasma; and lifting thesubstrate above the mid-lift position to an upper position at which thesubstrate can be removed from the plasma chamber.
 2. The method of claim1, wherein: (a) the process gas is argon, nitrogen, or a mixturethereof; (b) the substrate is held in the mid-lift position for 2 to 30seconds; and/or (c) the substrate is lifted 0.5 to 3 mm above thesupport surface of the electrostatic chuck when in the mid-liftposition.
 3. The method of claim 1, wherein the substrate is lifted bylift pins made of electrically insulating material while dechucking anda voltage is applied to the electrostatic chuck.
 4. The method of claim1, wherein: (a) the substrate is a silicon wafer bonded to a glasscarrier, the method further comprising plasma etching the siliconmaterial; (b) the substrate is a semiconductor wafer, the method furthercomprising etching a silicon layer in the wafer; or (c) the substrate isa semiconductor wafer having at least one layer of dielectric material,the method further comprising plasma etching openings into thedielectric material.
 5. The method of claim 5, wherein He gas issupplied to the underside of the substrate while plasma etching theopenings in the dielectric material, the method further comprisingterminating supply of the He gas before the substrate is raised to themid-lift position.
 6. The method of claim 1, wherein He gas is suppliedto the underside of the substrate and, before the substrate is releasedfrom the electrostatic chuck, supplying of He gas to the underside ofthe substrate is terminated.
 7. The method of claim 1, wherein He gas issupplied to the underside of the substrate, and further comprisingapplying a backpressure of He at 1 to 5 Torr to an underside of thewafer while the process gas is in a plasma state.
 8. The method of claim1, wherein voltage to the electrostatic chuck is set to a value prior tolifting the substrate sufficient to reduce a spike in substrate voltagepotential to below 25V in magnitude.
 9. The method of claim 1, whereinthe substrate is lifted to at least 5 mm above the support surface ofthe electrostatic chuck during the lifting to the upper position. 10.The method of claim 1, wherein: (a) the plasma processing chamber is acapacitively coupled plasma processing chamber wherein an uppershowerhead electrode is located opposite a lower electrode on which thesubstrate is supported, and the energizing comprises supplying radiofrequency power to the lower electrode; or (b) the plasma processingchamber is an inductively coupled plasma processing chamber comprising acoil, and the energizing comprises supplying RF power to the coil. 11.The method of claim 10, wherein the chamber is the capacitively coupledplasma processing chamber and a gap between the upper electrode and thelower electrode is at least 20 mm and pressure in the chamber is at 15to 500 mTorr.
 12. The method of claim 1, further comprising applying adechucking voltage prior to extinguishing the plasma.
 13. The method ofclaim 12, wherein the dechucking voltage is set to a value within 50V orwithin 200V of a plasma-induced bias on the substrate.
 14. The method ofclaim 1, further comprising processing the substrate prior to liftingthe substrate, wherein the processing comprises (a) forming a layer onan upper surface of the substrate or (b) stripping photoresist from thesubstrate.
 15. The method of claim 1, wherein the lifting to themid-lift position and the lifting above the mid-lift position bothcomprise lifting with a pneumatic actuator, and wherein gas flow througha shuttle valve contributes to a low lifting force.
 16. The method ofclaim 1, wherein the lifting is carried out with a pneumatic liftmechanism comprising: a housing having an upper chamber and a lowerchamber; an upper piston slidably mounted to move up and down in theupper chamber; a lower piston slidably mounted to move up and down inthe lower chamber, the lower chamber comprising a hard stop defining anupper limit of travel of the lower piston; the three positionscomprising (1) a lower position at which the upper piston positions thelift pins below the upper surface of the electrostatic chuck, (2) themid-lift position at which the lower piston is in contact with the hardstop and a shaft of the lower piston raises the upper piston, and (3)the upper position at which the substrate can be removed from the plasmachamber; and wherein the upper piston and the lower piston areindependently pneumatically operated.
 17. The method of claim 16,wherein the lift mechanism includes a lift pin yoke operably connectedto a shaft extending upward from the upper piston.
 18. The method ofclaim 16, wherein: the housing includes gas inlets operably connected topressurized gas sources, the gas inlets including a first gas inlet influid communication with a portion of the upper chamber above the upperpiston, a second gas inlet in fluid communication with a portion of theupper chamber below the upper piston, a third gas inlet in fluidcommunication with a portion of the lower chamber above the lowerpiston, and a fourth gas inlet in fluid communication with a portion ofthe lower chamber below the lower piston; and an up force on the upperpiston is limited by an opposing pneumatic pressure provided bypressurized gas supplied to the first inlet.
 19. The method of claim 18,wherein: the first gas inlet is in fluid communication with a first gassource supplying gas at a pressure of 25 to 65 psig and the second,third, and fourth gas inlets are in fluid communication with a secondgas source supplying gas at a pressure of 70 to 120 psig, and the liftmechanism further comprises a controller operable to selectively supplywhich selectively supplies gas from the first and second gas sources tothe gas inlets.
 20. The method of claim 19, wherein the controlleroperates a valve operating to place the first gas inlet in fluidcommunication with either the first gas source or the second gas source.